Production of oxygen by rectification of air involving precooling the air



' 1952 M. J. CICALESE 2,584,985

PRODUCTION OF OXYGEN BY RECTIFICATION OF AIR INVOLVING PRECOOLING THE AIR Filed April 8, 1948 2 SHEETS-SHEET 1 IN V EN TOR. Mikael I finale.

2 KITTORNEY Feb. 12, 1952 J. CICALESE 2,584,985

PRODUCTION OF OXYGEN BY RECTIFICATION OF AIR INVOLVING PRECOOLING THE AIR Filed April- 8, 1948 2 SHEETS-SHEET 2 Patented Feb. 12, 1952 PRODUCTION OF OXYGEN BY RECTIFICA- TION OF AIR- ]NVOLVING PRECOOLING THEAIR Michael J. Clcalese, Forest Hills, N. Y., assignor to Hydrocarbon Research, Inc., New York, N. Y., a corporation of New York 7 Application April 8, 1948, Serial No. 19,787

8 Claims.

This invention relates to the production of oxygen by the liquefaction and rectification of air, and more particularly to an improved method of cooling the air introduced into the rectification system.

All temperatures herein are in degrees F. and pressures in pounds per square inch gauge.

Oxygen is commercially produced by the liquefaction of air and rectification of the liquefied air at low temperatures. The refrigeration necessary for air liquefaction is supplied to the air after it has been compressed and water cooled to approximately 70 to 110 F. by indirect heat exchange with the eiiluent products of rectification. The temperature to which the air is water cooled usually depends on the season of the year; thus in summer the temperature is usually about 110 F. and in winter about 70 F. To refrigerate the air, reversing heat exchangers of the regenerative or recuperative type of exceptionally high heat transfer capacity are employed through which the incoming air and the cold products of rectification are passed with periodically reversed operation so that streams of relatively warm air are flowed through the paths through which the nitrogen and oxygen traversed during the previous step of the process. The high-boiling impurities deposited in these paths during the passage of air therethrough are removed by sublimation during the subsequent fiow of the products of rectification in a reverse direction. Such reversal of flow may take place every three minutes. These exchangers represent a material and substantial part of the cost of an oxygen plant.

It is an object of the present invention to provide a process of producing oxygen by the liquefaction and' rectification of air in which a substantial saving in reversing exchanger volume, and thus a substantial saving in capital cost for a given oxygen capacity, is effected.

Another object is to provide a process for producing oxygen in the operation of which a lower pressure drop in the rectification product streams flowing from the rectification system takes place with consequent economy of power for compressing the air to be rectified.

Other objects and advantages of this invention will be apparent from the following detailed description thereof.

In accordance with this invention, air compressed, for example, to a pressure of about 60 to 100 pounds, preferably about '70 to 85 'pounds, and water-cooled, as customary, from a temperature of 250 to 400 F. to a temperature of from 70 to 110 F. before introduction into a reversing exchanger, is first passed through a non-reversing exchanger in indirect heat exchange relation with a stream of rectification product, which leaves the reversing exchanger at a temperature of from 40 to 55 F. and a relatively low pressure, usually from about 1 to about 3 pounds. At spaced points along the path of fiow of the rectification product stream, water is Introduced thereinto to efiect cooling of the rectification product stream by vaporization of the water. The cold content of this stream is imparted to the air stream flowing in indirect heat exchange relation therewith. The air is thus cooled to a temperature of about 45 to 65 F. before it enters the reversing exchanger.

The withdrawal of the rectification product stream, e. g.. nitrogen, from the reversing exchanger at a temperature of from 40 to 55 F. is an important and critical feature of this invention. If the rectification product is withdrawn at a lower temperature than 40 F. it has little capacity to evaporate water and hence little capacity to pick up refrigeration. If, on the other hand, it is withdrawn at a higher temperature, say 65 F., it will result in the air leaving the non-reversing exchanger at a correspondingly higher temperature. I have found that by withdrawing the rectification product stream at a temperature of from 40 to 55 F. and introducing water into this stream which is at a low pressure, materially below that of the incoming air stream, maximum refrigeration is imparted to the rectification product by evaporation of water, which refrigeration is employed to efiect cooling of the air. Optimum air cooling is thus effected in the non-reversing exchanger with consequent maximum possible saving in reversing exchanger costs, as will be explained more fully below.

As the rectification product fiows through the non-reversing exchanger in indirect heat exchange relation with the air, it gives up its cold content to the air. The rectification product is thus warmed. The spaced introduction of water effects cooling of the rectification product along its path of fiow. The cold thus produced is imparted to the air stream as the rectification product and air fiow in indirect heat exchange,

relation through the non-reversing exchanger.

The introduction of the air at a temperature of from 45 to 65 F. throughout the operation of the reversing exchanger, and irrespective of the season of the year, materially reduces the thermal load on these exchangers which, when operated in accordance with prior customary practice, had to be designed to efiect coolin of the air from a temperature of about F. Hence, the reversing exchangers for practicing this invention may be designed to eifect approximately 50 less cooling of the air with consequent savthe air.

3 ing in the size or capacity or the costly reversing exchangers. Furthermore, the pro-cooling oithe air is eil'ected in relatively simple and inexpensive equipment employing the available rectification product or products under conditions resulting in a'minimum pressure drop in the rectification product streams flowing from the rectificasprayed thereinto from line 22.

tion system with consequent improvement in the operation of the rectification system. Since the reversing exchangers are made smaller through this invention, reversal losses are naturally smaller. Also, because some of the moisture has been removed from the air during passage through the non-reversing exchanger, reversals can be eflected less frequently, than every 3 minutes.

mthe accompanyingdrawings forming a part of this specification and showing, for purposes of exempliflcation, preferred layouts of the equipment iorpracticing the process oi! this invention;

Figure 1 illustrates diagrammatically a preierred layout or equipment for practicing the process or this invention; and

Figure 2 illustrates another form of air coolingequipment hich may be used with this invention.

It will be understood that the invention may be carried out in other apparatus than that shown in the drawings; for example, any desired number of reversing exchangers may be used in lieu of the reversing exchangers shown in the drawings; each of the reversing exchangers shown in the drawings may be replaced by two or more smaller exchangers placed in series and/or parallel, if desired; other rectification systerm may be used in lieu of that shown in Figure 1. etc.

Referring to Figure 1, ll is an air cooling tower of any well known type into which air under pressure of 60 to 100 pounds and at a temperature of 250 to 400 F. is introduced at H and passes upwardly countercurrent to cooling water introduced at 12 and exiting from the tower at 13. The air leaves the washing tower in through line I entering a non-reversing exchanger 15.

In the embodiment of the invention shown in Figure 1, the air flows from exchanger 15 through line I. to exchanger l1, thence by way or line ll to exchanger II and discharges into line 20. Nitrogen flows in indirect heat exchange relation with the air passing through these exchangers. The nitrogen flows from line 43 through humidiiier 2|, line 22, exchanger 12, line 23, humidifier 21', line 24, exchanger l1, line 25, humidifier 2!", line 28, exchanger II and exits through line 21. In each or exchangers 15,11 and is the nitrogen flows in indirect heat exchange relation with The humidifiers 2|, 2| and 2|" may be chambers through which the nitrogen flows in direct contact with a water spray which acts to depress the temperature or the nitrogen in its flow therethrough. Other types or humidifiers like towers packed with slats over which the water flows downwardly against a rising gas stream may be employed. While three humidiners are shown in Figure 1, any desired number can be used to eii'ect optimum cooling oi the nitrogen stream.

The air on passing through non-reversing exchangers l5, l1 and i9 is progressively cooled so that some of its moisture is condensed. water traps lia, Ila and a. are used to separate the condensed water which is discarded at 16b, itb and 2lb, respectively.

In the embodiment shown in Figure 1 each of say every 5 minutes rather the humidifiers is a vessel through which the nitrogen flows in direct contact with water The water which the nitrogen collects at the drains therefrom through 30 which forces the Makeis not vaporized by bottom or the vessel, line 20 equipped with pump water through line 2| back into the vessel. up water is introduced through line I i The air, thus cooled to a temperature or from 45 to F., flows into an air line 33 provided with branches 3|, It leading into the reversing valves 30, 31 associated with the reversing nitrogen-air exchanger II and the oxygen-air exchanger respectively. These exchangers may be otany well known type. In the embodiment shown in the drawing they consist of a shell in which are provided three flow paths. In the case oi! exchanger 38, concentric paths a, 40 and 41 are disposed in heat exchange relation with each other. In the case of exchanger :9, concentric paths l2a, 42 and 43, likewise, are disposed in heat exchange relation with each other. Each heat exchanger has in its paths suitable fins 01 heat conducting material, e. g., copper or aluminum, promoting rapid and eflicient heat exchange between the gaseous media flowing therethrough.

For purposes of illustration, and in the interests of simplicity, each flow path in an exchanger is shown on the drawings as consisting of a single tube, the paths being disposed concentrically. However, each path in the exchanger may comprise a multiplicity of tubes for flow therethrough.

Paths l0 and ll are the paths through which air and nitrogen flow, the flow or these two media through their respective paths being periodically reversed so that during one step of the process air flows through path ti and nitrogen through path 40 and upon reversal during the succeeding step air flows through path 40 and nitrogen through path 41. Reversal of flow is accomplished by suitably positioning the reversing valves 36, 44 which may be of any well known type. Valve 36 is disposed in a pipe line system consisting of the air inlet branch 34 leading into valve 36, nitrogen exit line 45 communicating with humidifier 2| and the non-reversing heat exchanger I9, and pipe lines 46. 41 leading to one end of paths 40, ll, respectively. Pipe lines 48, 49 lead from the other end of the exchanger paths 40, ll, respectively, to valve 44. Air line 50 leads from valve 44 and nitrogen inlet line 5| leads into this valve.

Paths 42 and 43 are the paths through which air and oxygen now, the flow of these two media through their respective paths being periodically reversed so that during one step of the process air flows through path 43 and oxygen through path 42, and upon reversal during the succeeding step air flows through path 42 and oxygen through path 43. Reversal of flow is accomplished by suitably positioning the reversing valves 31, 52, which may be of any well known type. Valve 31 is disposed in a pipe line system consisting of the air inlet branch 35 leading into valve 31, oxygen exit line 53 and pipe lines 54 and 55 leading into one end of paths 42, 43, respectively. Pipe lines 56 and 51 lead from the other end of the exchanger paths 42, 43, respectively, to valve 52. Air line 58 leads from this valve and an oxygen inlet line 59 leads into this valve.

Reversing exchangers 33 and 39 may be placed in vertical, horizontal, or any other desired position. When these exchangers are arranged vertically, the cold end may be above or below the warm end. In general, the nitrogen flow paths through the exchangers should have approximately four times the volumetric capacity of the oxygen flow paths. If desired, exchangers in which the oxygen and nitrogen flow paths are of the same volumetric capacity may be employed, in which case four air-nitrogen reversing exchangers are employed for each air-oxygen reversing exchanger. Also, of the total air cooled by indirect heat exchange with the nitrogen and oxygen products of rectification, about 80% flows through the air-nitrogen exchanger 36 and about 20% through the air-oxygen exchanger 39.

The rectification system 60 comprises two columns 6| and 62. Column 6| is operated at a pressure of from about 60 to 100 pounds, preferably at about '70 to 85 pounds, and column 6.2 at a pressure of from about 2 pounds to about 10 pounds, preferably at about 5 pounds. These columns, as customary, are provided with rectification plates of the bubble cap or other desired type. The cooled air issuing from lines 50 and 58 passes into line 63 whence a major portion of the air flows to line 64 passing through nonreversing exchangers 68 and 69 and discharging into the base portion of column 6|. The remainder of the air passing through line 63 flows partly through line 66 and valve 66a and partly through line 65, valve 65a, path 40a in exchanger 38, line 40b and into line 6617 where it mixes with the cooled air flowing through valve 66a. The resultant mixture enters expander 61 at a temperature such that no liquefaction takes place upon expansion in this expander. Crude oxygen containing approximately 40% oxygen, the rest being chiefly nitrogen, flows from the base of column 6| through line 10 which passes through a non-reversing heat exchanger 1|. Upon flow through the expansion valve 12 in line HI the crude oxygen is flashed, entering column 62 at 13. Line M leads from the top of column 6 passes through a non-reversing heat exchanger 15 into a line having one branch 16 for returning liquid reflux comprising chiefly nitrogen into column 6| and another branch 11 passing through the non-reversing exchanger 18 and leading into the low pressure column 62 at 19. An expansion valve 80 is disposed in branch 11.

Air from expander 61 fiows through a line 8| into the low pressure column 62 at a level below point 13. The base of this column is provided with a line 82 leading into the non-reversing heat exchanger 15. A line 83 leads from this exchanger into the low pressure column 62. The lines 82, 83 and the cooperating heat exchanger I5 provided with a valve Ma and with a. branch line 88 equipped with a valve In and leading into path 42a of exchanger 39. A line 89 leads from the exit end of path 42a into line 6| as shown in Figure 1. A minor portion 01 the nitrogen warmed by passage through path 42c flows through line II and it mixes with the remainder of the nitrogen passing through line 5| into reversing valve l2 An oxygen line 61 leads from the base of low pressure column 62 into the exchanger 68 through which the air passes. From this exchanger oxygen line 59 leads into the reversing valve 62.

As a specific example of the process of this invention in the equipment shown in Figure 1, air compressed to a pressure of about 85 pounds at a temperature of about 335 F. fiows up through the air coolingtower Ill counter-current to a downflowing stream of water. The air is thus cooled to a temperature of about 105 F. The air at this temperature fllows through the non-reversing exchanger |5 countercurrent to a stream of nitrogen entering at a temperature of 63 F. and leaving at a temperature of 90 F. The air exists at a temperature of 92 F. and at this temperature enters function as a reboiler; liquid oxygen flows through line 82 into exchanger 15 in indirect heat exchange relation with the gaseous stream comprising chiefly nitrogen passing through line 14 which causes vaporization of the liquid oxygen to take place, the oxygen vapors flowing into column 62. Line 82 is provided with valved branch 82a for periodically removing some liquid oxygen to prevent the build-up of high-boiling impurities of air like acetylene within the rectification system.

A nitrogen line 84 leads from the top of column 62 into exchanger 18. From this exchanger, a nitrogen line 85 leads into exchanger 1| through which the crude oxygen passes as hereinabove described. Fron exchanger 1| a nitrogen line 86 leads to exchanger 69 through which the air passes. Nitrogen line 5| leads from exchanger 69 into the reversing valve 44. Nitrogen line 5| is exchanger H. The air flows from exchanger H at a temperature of 77 F. into exchanger l9 and thence at a temperature of 60 F. into line 33. The nitrogen flowing through these non-reversing exchangers countercurrent to the air in indirect heat exchange relation therewith passes from reversing exchanger 38 at a temperature of 51.5 F. and a pressure of about 1 pound into humidifier 2| where through the vaporization of water the temperature of the nitrogen is reduced to 38 F. At this temperature the nitrogen enters exchanger |9, passes thence at a temperature of 65.5 F. into humidifier 2|, issues at a temperature of 52 F., passes through exchanger l1 leaving at a temperature of 79 F., enters humidifier 2|" and issues at a temperatureoi 63 F. The

air in its flow through non-reversing exchangers l5, l1 and IS in indirect heat exchange relation with the nitrogen is thus cooled to a temperature of 60 F. Approximately 20% of the thus cooled air flows through air branch 35 and the remaining through branch 34. The air from branch 34 flows into reversing valve 36, line 41, path 4| in indirect heat exchange with the nitrogen flowing from valve 44 through line 46, path 40 into line 46, the nitrogen leaving path 40 at a temperature of 51.5 F. and at this temperature entering humidifler 2| through line 45. The air in its flow through path 4| is cooled to a temperature of about 274 F. by the nitrogen entering at a temperature of about 282 F. Simultaneously, air flows from branch 35 through valve 31, line 55 into and through path 43 of exchanger 39, leaving this path through line 51 at a temperature of about 274 F. Oxygen at a temperature of about 282 F. flows from line 59 through valve 52, line 56 into and through path 42, leaving this path through line 54, flowing through valve 31 and exiting through line 53 at a temperature of 51.5 F.

About. 20% of the cooled air flows in part through path 40a into line 66b and the remainder through line 66 into line 66b. The resultant mixture, warmed to a temperature of about 240" F. flows into expander 61 and is expanded to a pressure of about 7 pounds. At this pressure and at a temperature of about 305 F. the expanded air enters low pressure column 62 through line 8|. The remaining major portion of the air flows throughline 64 into and through heat exchangers 68 and 68 and into the base of the high pressure column SI. The air is rectified in the rectification system 60 to produce the oxygen and nitrogen products of rectification, the oxygen product of rectification flowing from low pressure column 82 through lines 81 and 59 into the reversing exchanger 39 and the nitrogen product of rectification flowing from the top of column 62 through lines 84, 85, 8'5 and 5|, a portion of the nitrogen flowing through line 88, path 421: and line 39 mixing with the remainder of the nitrogen passing through line 5| into the reversing exchanger 38.

Upon reversal, which may take place every five minutes, air from branch 34 flows through valve 36. line 45 and flow path 40 of the heat exchanger 33 while nitrogen fiows through flow path 4| of this exchanger. Air from branch 35 flows through valve 31, line '54 and fiow path 42 of heat exchanger 39, while the oxygen flows through flow path 43. The air is thus cooled to a temperature close to. its dew point as hereinabove decsribed. In this step of the process, fiow through the non-reversing exchangers I5, I1 and I9, paths 40a and 42a of exchangers 38 and 39, respectively, expander 91 and the rectification system 90 is the same as hereinabove described in connection with the preceding step. The nitrogen flowing through flow path 4I removes by sublimation condensibles such as moisture and carbon dioxide deposited therein from the air stream flowing therethrough during the preceding step of the process. In like manner, the oxygen flowing through flow path 43 removes therefrom condensibles deposited therein from the air stream passing therethrough during the preceding step of the process. There is relatively little pressure drop in the flow of the nitrogen and oxygen through the air cooling system.

The modification of Figure 2 differs from that of Figure 1 in that a regenerative type of reve sing heat exchanger is used instead of the recuperative type of Figure 1, a tubular type of non-reversing heat exchanger provided with water inlets is used for cooling the air prior to introduction into the reversing exchangers and the air is pre-cooled by passage in indirect heat exchange relation with both the nitrogen and oxygen products of rectification instead of with only the nitrogen as in Figure 1.

In Figure 2 the air under pressure and after cooling in a water-cooled exchanger 9 corresponding to tower l of Figure 1 flows through line I4 into branches I00 and I0l, communicating with the non-reversing exchangers I02 and I03. Each of these exchangers desirably is of tubular construction consisting of tubes I04 connecting headers I05 and I08 at the opposite ends of the exchangers. The air flows through the tubes in indirect heat exchange relation with the rectification products passing through the shell side of the exchangers in heat transfer relationship with the tubes. Exchanger i03 is provided with an oxygen inlet line I01 and an oxygen exit line I08 at the opposite end. The general direction of flow of the air through exchanger I03 is countercurrent to that of the oxygen, the air being admitted at the end of this exchanger where the oxygen exits and the air withdrawn where the oxygen enters. At spaced points along the shell side of the exchanger water is injected through the jets I09 leading from awatermain H0. The amount of water thus introduced through each jet I09 is such the oxygen is transferred to the air, the oxygen thus warmed in its continued flow through the exchanger being again chilled by the injection of water through a succeeding water jet I09, etc.

Non-reversing exchanger I02 is approximately four times the capacity of exchanger I03 and has a nitrogen inlet I II and a nitrogen exit line H2. The general direction of flow of the nitrogen through exchanger I02 is countercurrent to that of the air, the nitrogen passing about the outside of tubes I04 through which the air passes. At spaced points along the shell side of exchanger I02 controlled amounts of water are introduced through jets II3 communicating with a water main H4. The amount of water thus introduced through each jet is such that the water is vaporized by the nitrogen.

The air in its flow through the exchangers I02, I03 is cooled to a temperature of 45 to 65 F. by the oxygen and nitrogen entering at a temperature of 40 to 55 F. The thus cooled air Grits from exchanger I02 through line H5 and from exchanger I03 through line II5, both lines H5 and H8 leading into the main air line II1 communicating with the reversing valves I I3 and H9.

The cooled air passing through line II1 flows through water trap Illa which eliminates condensed water at II1b. While the water injected into exchangers I02 and I03 may be fed at a rate such that it completely vaporizes in the rectification product streams passing through these exchangers, preferably these exchangers are provided with valved lines I02a and IBM so that excess water may be used to insure saturation of the rectification product streams with water and the excess water drained through these valved lines; the withdrawn water may be returned to the inlet lines H4 and I III. The water injected into a rectification product stream to effect cooling thereof should preferably be at a temperature not substantially higher than that of the rectification product stream. Ordinary tap water temperatures are generally satisfactory.

The regenerator system of Figure 2 comprises two regenerator groups I20 and IN. Regenerator group I20 comprises two regenerator pairs I22, I23 and I24, I25 for the alternate flow'of air and nitrogen therethrough. Regenerator I22 communicates with I23 through line H211 and I24 with I25 through line I24a. Regenerator group I comprises two regenerator pairs I29, I21 and I20, I29 for the alternate flow of air and oxygen therethrough. Regenerator I23 communicates with I21 through line |26a and I28 with I29 through line I28a. Desirably each of the regenerators of group I20 is approximately four times the capacity of each of the regenerators of group I2I. Each regenerator unit may be of any well known type containing heat transfer material of high heat absorbing capacity; for example, they may contain packing units of copper or aluminum as disclosed and claimed in pending application Serial No. 783,498, filed November 1, 1947.

Reversal of flow through the regenerator group I20 is accomplished by a pair of reversing valves H8 and I35. Reversing valve II8 communicates with regenerators I22 and I24 through pipe lines I31 and I38 and is provided with an air inlet line leading from air line H1 and a nitrogen exit line III. Reversing valve I39 communicates with regenerators I23, I25 through lines I39 and I40, respectively, is provided with an air exit line I50 and a nitrogen inlet line 5I corresponding with 9 the line bearing the same reference numeral in the modification of Figure 1 and leading from rectification system 60,such as shown in Figure 1, or .any other well known type of rectification system in which the refrigerated air is rectified to produce oxygen and nitrogen.

Reversal of fiow through regenerator group IZI isaccomplishecl by a pair of reversing valves H9 and MI. Valve H9 is provided with lines I42 and I43 leading into the regenerators I26, I28, respectively, is also provided with an air inlet line leading from the main air line I I1 and the oxygen exit line I01. Reversing valve HI communicates with the regenerators I21, I29 through lines I44, I45, respectively, is provided with an oxygen inlet line 59 and an air exit line I5I. Air outlet lines I50 and I5I discharge into line 53 whence the major portion of the cooled air flows to line 64 and rectification system 50. The remaining minor portion of the air fiows through line 65 and valve 65a and thence in two equal streams through tubes I23a and I25a in regenerators I23 and I25, respectively. The air fiows through tubes I23a and I25a in indirect heat exchange relation with the nitrogen and air passing through regenerators I23 and I25. The minor portion of the air is thus warmed so that on flowing through line I52 to expander 51 the formation of liquid air during expansion is avoided. The expanded air flows through line 8| to column 62 as hereinbefore described.

To maintain a small temperature difference of from about 5 to F., preferably about 6 to 8 F., between the air exiting regenerator group IZI and the oxygen entering this group, part of the oxygen fiowing through line 59 is diverted through branch line I53 and thence in two equal streams through tubes I21a and I29a of regenerators I21 and I29, respectively. The oxygen fiows through tubes I21a and H91: in indirect heat exchange relation with the oxygen and air passing through the regenerators I21 and I29 on the outside of these tubes. The thus warmed oxygen passes through line I54 and reenters line 59 between valves 59a and MI. The minor portion of the air which is warmed by passage through tubes I23a and I25a, similarly, serves to maintain the small temperature difference between the air leaving regenerator group I20 and the nitrogen entering this group.

In the modification of Figure 2 streams of nitrogen rectification product and air, pre-cooled to a temperature of from 45 to 65 F. by fiow through exchangers I02 and I03, flow alternately over the packing disposed in the regenerator pairs I22, I23 and I26, I25, the nitrogen imparting its cold content to this packing and the air recovering this cold when it fiows thereover during the succeeding step of the process. Oxygen and air pre-cooled by flow through exchangers I02 and I03 fiow alternately through the regenerator pairs I26, I21 and I28, I29, the oxygen imparting its cold content to the packing in these regenerators and the air recovering this cold when it flows thereover during the succeeding step of the process. During flow of the nitrogen through one Or the other of the regenerator pairs and the flow of the oxygen through one or the other of the regenerator pairs, these rectification products remove by sublimation condensibles deposited in these regenerators from the air in its fiow therethrough during the preceding step of the process. Thus, in the continued operation upon each reversal the nitrogen and oxygen rectification products effect removal of t carbon dioxide, moisture and other condensibles deposited in the paths through whichthe air had passed during the preceding step of the process thereby permitting continuous operation. The pre-cooling of the air permits use of regenerators of materially smaller volumetriccapacity for a given oxygen plant capacity and thus materially reduces plant cost for a given oxygen capacity.

It will be noted the process of this invention permits the use of less costly reversing heat exchangers for a given capacity of oxygen production and also results in an economy of air compression power in that lower pressure drop takes place in the flow of rectification products from the rectification system through the air cooling system than is the case in other known air cooling systems.

The expressions reversing the flow of air and nitrogen" and reversing the flow of air and oxygen and reversal" are used herein in the sense commonly employed in this art, namely, to mean the switching of the flow of the two streams, for example, the air and the nitrogen streams, or the air and the oxygen streams, so that upon each reversal the air flows through the path through which had previously flowed the oxygen or nitrogen and the nitrogen or oxygen fiows through the path through which had previously flowed the air.

In the claims the expression introducing water into a rectification product stream is intended to mean the addition of water to the rectification product stream either by passing the rectification product lnto contact with water or by injecting the water directly into the rectification product or otherwise so that evaporative cooling of the rectification product takes place.

Since certain changes in carrying out the above process and different embodiments of the invention can be made. without departing from the scope of this invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The improvement in the process of producing oxygen by the liquefaction and rectification of air involving the fiow of air under pressure of 60 to pounds per square inch gauge through reversing exchangers in indirect heat exchange relation with nitrogen and oxygen products of rectification and thereafter subjecting the thus cooled air to rectification to produce 7 said nitrogen and oxygen products of rectification, which improvement comprises passing the air at a pressure of 60 to 100 pounds per square inch gauge through non-reversing exchangers in indirect heat exchange relation with streams of nitrogen and oxygen rectification products prior to introducing the air into the reversing exchangers, said nitrogen and oxygen leaving said reversing exchangers at a temperature of 40 to 55 F., and introducing water into said streams of nitrogen and oxygen rectification products at a plurality of spaced points along their paths of flow through said non-reversing exchangers to effect cooling of said streams of nitrogen and oxygen rectification products through vaporization of water and thereby to cool the air in itsv flow through said non-reversing exchangers to a temperature of 50 to 60 F.

2. The process of producing oxygen by the liquefaction and rectification of air comprising scrubbing with water a compressed air stream at a pressure of from 60 to 100 pounds and at a temperature of from about 250 to about 400 F., thereby cooling the air to a temperature of from 70 to 110 F., flowing the air from the water scrubbing treatment through a non-reversing exchanger in indirect-heat exchange relation with a stream of nitrogen rectification product leaving a reversing exchanger at a temperature of 40 to 55 F., introducing water into the nitrogen stream at spaced points along its path of fiow through the non-reversing exchanger to effect cooling of said stream of nitrogen through vaporization of water and thereby to effect cooling of the air so that it leaves said nonreversing exchanger at a temperature of 45 to 65 F., passing the air at a temperature of 45 to 65 F. through a path in said reversing exchanger to recover the cold content of the nitrogen product of rectification, and introducing the air into a rectification system where the air is rectified to produce the nitrogen product of rec tification.

3. Theprocess of producing oxygen by the liquefaction and rectification of air comprising scrubbing with water a compressed air stream at a pressure of from 60 to 100 pounds and at a temperature of from about 250 to about 400 F., flowing the air from the water scrubbing treatment through a non-reversing exchanger in indirect heat exchange relation with a stream of nitrogen rectification product leaving a reversing exchanger at a temperature of 40 to 55 F., introducing water into the nitrogen stream at spaced points along its path of fiow through the non-reversing exchanger to effect cooling of said stream of nitrogen through vaporization of water and thereby to effect cooling of the air so that it leaves said non-reversing exchanger at a temperature of 45 to 65 F., passing a portion of the air at a temperature of 45 to 65 F. through a path in a reversing exchanger to recover the cold content of the nitrogen product of rectification, passing the remainder of the air through a path in a second reversing exchanger to recover the cold content of the oxygen product of rectification, introducing the air from both of said reversing exchangers into a rectification system where the air is rectified to produce the oxygen and nitrogen products of rectification, and periodically reversing the flow of the air and nitrogen in the first-mentioned reversing exchanger so that the nitrogen fiows through the path in the first-mentioned exchanger through which path the air had previously passed thereby removing from this path condensibles deposited therein from the air stream passing therethrough during the preceding step of the process and the oxygen fiows through the path in the secondmentioned reversing exchanger through which the air had previously passed thereby removing therefrom the condensibles deposited therein from the air passed therethrough during the preceding step of the process.

4, A process of producing oxygen as defined in claim 3, in which the reversing exchangers are of the recuperative type.

5. A process of producing oxygen as defined in claim 3, in which the reversing exchangers are of the regenerative type.

6. The improvement in the process of producing oxygen by the liquefaction and rectification of air, which improvement comprises passing the air through a series of non-reversing heat exchangers in indirect heat exchange relation with a stream of rectification product at a lower temperature than the temperature of the air stream and alternately saturating and partially desaturating wid stream of rectification product with respect to its content of water vapor, the saturation of said stream of rectification product with consequent cooling thereof being effected by introducing water thereinto during its flow between each pair of nonreversing exchangers and the partial desaturation of said stream being effected during its fiow through each of said non-reversing heat exchangers in indirect heat exchange relation with the air flowing therethrough thus cooling said air and warming said stream of rectification product.

7. The improvement in the process of producing oxygen by the liquefaction and rectification of air involving the flow of air under pressure through a reversing exchanger to recover the cold content of a nitrogen product of rectification and thereafter subjecting the thus cooled air to rectification to produce oxygen and said nitrogen product of rectification, which improvement comprises passing the air through a series of non-reversing exchangers in indirect heat exchange relation with a stream of nitrogen rectification product leaving said reversing exchanger at a temperature of 40 to 55 F., saturating said stream of nitrogen rectification product and thus cooling it by the introduction of water thereinto prior to its introduction into the first exchanger of said series of non-reversing exchangers, the saturated nitrogen stream being partially desaturated with respect to its content of water vapor in its flow through each of said non-reversing exchangers and being again saturated and thus cooled by the introduction of water thereinto during its fiow between each preceding non-reversing exchanger and each succeeding non-reversing exchanger thereby cooling the air in its flow through said series of non-reversing exchangers to a temperature of 45 to 65 F.

8. In the process of producing oxygen by the liquefaction and rectification of air involving the flow of air through a reversing exchanger to recover the cold content of a rectification product and thereafter subjecting the thus cooled air to rectification, the improvement which comprises precooling the air, prior to introducing said air into said reversing exchanger, by indirect non-reversing heat exchange across a heat transfer wall with a stream of gaseous rectification product coming from said reversing exchanger at a temperature of about 40 to 55 F., and introducing water into said stream of gaseous rectification product in the course of said indirect non-reversing heat exchange to humidify and thus cool said stream of gaseous rectification product and thereby'to precool said air by said indirect non-reversing heat exchange to a temperature of 45 to 65 F.

MICHAEL J. CICALESE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,724,513 Pollitzer Aug. 13, 1929 2,141,997 Llnde et al Dec. 27, 1938 

